Problem: If $x \triangleleft y = x(y-4)$ and $x \oplus y = x-4y$, find $(2 \triangleleft 5) \oplus -1$.
Answer: First, find $2 \triangleleft 5$ $ 2 \triangleleft 5 = 2(5-4)$ $ \hphantom{2 \triangleleft 5} = 2$ Now, find $2 \oplus -1$ $ 2 \oplus -1 = 2-(4)(-1)$ $ \hphantom{2 \oplus -1} = 6$.